Silicon nitride (Si3N4) plays an important role in semiconductor manufacturing and serves to form, for example, antireflective coatings, protective layers of IC chips, and dielectric films of capacitors. The oxidation rate of Si3N4 is by several orders of magnitude lower than that of silicon. This property is employed to generate various kinds of electric insulations by the method of LOCOS (local oxidation of silicon), which is described, for example, in S. Wolf and R. N. Tauber, “Silicon processing for the VLSI era”, volume 2, Latice Press. While plasma etching is the appropriate method for structuring a Si3N4 layer prior to field oxidation, it is not applicable after the oxidation because of its low selectivity between Si3N4 and silicon oxide.
A high selectivity can be achieved by means of wet chemistry. Hot phosphoric acid (H3PO4) is conventionally used to etch Si3N4. The chemical reactions are summarized in the following equation, presented in S. Clark, “Silicon Nitride Etch”, Bold Technologies technical notes:3Si3N4+27H2O+4H3PO44(NH4)3PO4+9H2SiO3.
According to this equation, water hydrolyzes the silicon nitride to form hydrous silica and ammonia, which remains in the solution in the form of ammonium phosphate. During the etching of the nitride layer, hydrated silicon dioxide (H2OSiO2) is formed, which inhibits the etching of SiO2. This results in a selectivity of 50:1 or higher between nitride and oxide.
The etch rates of Si, SiO2 and Si3N4 in phosphoric acid are investigated in the paper of W. van Gelder and V. E. Hauser, “The etching of silicon nitride in phosphoric acid with silicon dioxide as mask”, J. Electrochem. Soc. 114, p. 869 (1967). The higher the temperature and the higher the water content, the higher is the etch rate of silicon nitride. To obtain satisfying process results, it is not sufficient to control the temperature of the H3PO4; also the water content has to be maintained stable. Therefore the effect of the temperature on the acid concentration has also to be taken into account. Running the etch bath at a constant water content at the corresponding boiling point is a suitable means of maintaining stable conditions in the etch bath. The relationship between the concentration of the phosphoric acid and the boiling point temperature is shown in the diagram of FIG. 2, which was taken from the aforementioned paper.
A constant temperature can be maintained by regulating the acid concentration. This can be accomplished with a reflux system, in which the steadily evaporating water is condensed and fed back to the etch bath. As such a system is not closed, water must be added to ensure a stable water content of the phosphoric acid. As the hydrous silica generated in the etch process is responsible for the high selectivity but is not present from the very beginning of the etching process, the selectivity between nitride and oxide may increase from typically 25:1 to typically 100:1, when the etch bath reaches its saturation with SiO2.
U.S. Pat. No. 5,779,927 discloses a reflux etcher, in which the liquid acid evaporant is condensed and returned to the etch bath by way of a pH meter, whose output is used to control the flow of additional water into the main system. Alternatively, a conductivity meter, located in the etch bath, may be used for the same purpose.
U.S. Pat. No. 5,938,885 discloses a method for a continuous control of the etch rates by means of a gravity analysis of a sample stream from the H3PO4 solution in the etch bath chamber. Depending on the gravity analysis, water is added to the etchant or heat is supplied to reduce the water content.
U.S. Pat. No. 6,207,068 B1 discloses a silicon nitride etch bath system, in which a secondary filter that is operatively connected to a heat exchanger is used for extracting silicon dioxide particles in a small portion of phosphoric acid removed from an etching bath. The arrangement enables to prevent the concentration of SiO2 from reaching the saturation level.
US 2008/0035609 A1 discloses a method which controls the concentration ratio of the components dynamically on the basis of a particle count within the etchant and stabilizes the etch rate by adding both water and H2SO4.
US 2008/0066864 A1 discloses an etch apparatus, which makes use of a dissolver included in a recirculation path coupled to a tank. The dissolver comprises a porous carbon matrix filter that is coated with silicon nitride, which is dissolved into the etchant at a controlled dissolution rate.
EP 0 474 482 A1 discloses a method of etching silicon nitride with H3PO4, in which the temperature of the etchant is detected and a solution of H3PO4 of low acid concentration is added to the etchant if the temperature increases above a predetermined value.
WO 02/27310 A1 discloses a method of measuring the water content of a liquid mixture contained in a tank. The temperature of a cooling medium passing the tank is measured at an inlet and at an outlet. The temperature difference is used to compute the water content of the liquid mixture.